Furnace / AC / Air Exchanger Problem?

[AZRedhawk44]

My new house has two AC units that are both pretty old:  Mid 90's.

The furnace or air exchanger or whatever it's called... I have two of those as well.  One that serves the living room, kitchen and dining room is in a small closet.  The one that serves the bedrooms is up in the attic.

AC is set to 80 degrees.

System is set to "cool."

Fan is set to "auto."

Current temp in the house is 76 on one thermostat, 75 at the other.

The fan (not the AC unit outside) on the air exchanger for the back of the house is running.  I don't want it running.  The one on the front of the house isn't running... it's acting like it should.  How do I get the back one to shut off?

I've switched the system to "off" rather than cool, and there's no other setting for the fan besides "auto" and "on."  I switched it to "on" and then back to "auto" for a few minutes seeing if that would kick it back off.  No dice.

Help me, home owning gurus of APS!

[charby]

A relay or capassitor may have gotten loose on the fan. I wish I could be more techinical but the last place I lived either one would get loose in the furnace and I had to reseat it.

[Nick1911]

Also, I think some fan limit switches have a button on them for always on.

You might check this on that furnace.

http://www.inspect-ny.com/heat/Fan_Limit_Switch574-DFs.jpg

[AZRedhawk44]

Also, I think some fan limit switches have a button on them for always on.

You might check this on that furnace.

http://www.inspect-ny.com/heat/Fan_Limit_Switch574-DFs.jpg

This will be up in the attic... gotcha.  I'll go up and look for it this evening.

[Jim147]

Stuck fan relay, stuck t-stat.
Wires shorted together in wall between t-stat and unit. This is the most common one I see around here.
Try unhooking t-stat wires to see if it stops. Do you have an Ohm meter?


jim

[AZRedhawk44]

I do have an ohm meter, and I swapped the little Honeywell thermostat control modules between the front and back zones.

The fan stayed on in the bedroom zone.

What can I check with my ohm meter?

[Brad Johnson]

Turn the thermostat for that unit off then kill the breaker. Leave it off for a few minutes. Kick it back on. If the fan doesn't come back on then you probably have a flaky relay or contactor in the outdoor unit.

By the way, mid-90's isn't all that old for an HVAC unit. Average life for most units is 20-25 years.

Brad

[280plus]

Something is stuck. Or maybe the switch nick mentioned but if it only does A'C and has nothing to do with heat it won't have a fan/limit switch buolt in. I get the impression you're talking an A/C (cool) only "air handler" in which case my guess is stuck fan relay.

 Just FYI The fan "on/auto" switch, when it's in "on" the fan runs continuously when it is in "Auto" the fan only comes on when the heat or cool is running.

[AZRedhawk44]

The AC unit outside doesn't run when the fan runs... just the air exchanger fan.

Not saying the AC doesn't work, BTW... just that it only kicks on when the house temp is over the thermostat setting.

Just did a test.

1.  Turn off AC and heat at the thermostat for that AC/Exchanger unit.
2.  Turn fan to "auto" setting... which should be off if the AC and heat aren't running.
3.  Listen for fan:  Yep, it's running.
4.  Go out to power box, kill breaker for attic exchanger.
5.  Come back inside, listen for fan:  Nope, it turned off.  Sweet.
6.  Go out to power box, flip breaker back on for attic exchanger.
7.  Come back inside, listen for fan:  Nope, it's still off.  Sweet.
8.  Go to thermostat panel: It reads AC off, heat off, fan on auto.
9.  Flip fan to on:  Fan comes on.
10.  Wait a minute, flip fan to auto again:  Fan won't turn off unless I go back to the breaker and kill the breaker.

So:  bad fan relay?

[Jim147]

What can I check with my ohm meter?

You can check about anything with one. Just make sure you have some spare fuses for it in case it says you shouldn't have checked that.

I did have to think about this for a bit. I am used to talking service techs through something like this I guess I do have to worry about liability a little.

So here goes your crash course in low voltage HVAC.

When you say you swapped tsat's I'm guessing you have the ones that just snap off with some pins coming out the back to contact the backplate.

Pull the tsat off the the running unit. From your description it should stay running. If it stops your problem may be in the backplate or the wires to the backplate.

If it keeps running, go find your service switch or breaker to turn it off. Pull the backplate off the wall and check for broken or bare wires touching each other. As soon as you pull it off they stop touching so this may be hard to see.

If you don't find a problem there then you need to ohm between the wires. I have seen a lot of different color wires used on the low voltage side so I have to do more guessing that this is a standard red, white, green and yellow single transformer system.

With the power still off, go to the unit and find the red transformer wire and the green fan wire. I will say again that you may not have these colors. Unhook the two wires and ohm between them. It should be open. If you are reading the wires as closed you have a short in the wires somewhere between the unit and tsat.

If all of this checks good then you may have a relay problem. I would like to see how it is wired before I go to far into telling you what to check on that

jim

Just read your post as I was posting this. Get a wiring diagram so we can tell you how to check that fan relay.

[280plus]

So:  bad fan relay?

Not necessarily I'm afraid to say cause what you are describing sounds kind of wierd. Let me mull my brain on it for a while. It sounds like something I've seen before but I can't quite place my finger on what it is.

[280plus]

Meanwhile see what Jim comes up with for you.

[Jim147]

I have a few thoughts but I'm not sure what to tell you at this point if it goes away when you cut power but comes back when you turn it on.
I hate saying too much without knowing more about the unit you have. Do you know if it is a standard six terminal fan relay?

My mind is a little split right now. My FIL called right before my last post. He has no heat and the outside unit running with the heat on and it's not a heatpump. I've been out getting my tools and a  roll of low voltage wire. I'll be just down the street from his house tomorrow so I'll load up anything I can think of and a change of clothes.

Let us know if you have a wire diagram on the furnace or an owners manual you can put online.

jim

[Brad Johnson]

i'd still be tempted to check the in-unit fan relay/contactor. Sounds an awful lot like it's hung in the closed position. With some units it's handing right out in the open. On most it's behind a small cover panel you can access when you pull off the main access door. Hopefully there will be a diagram taped somewhere on the unit or stuck inside the access panel for the control board.

Brad

[280plus]

How is the filter in that thing? I want to say it sounds like a manual reset high temperature limit is tripped and that could be caused by a dirty filter. Some units will energize the fan when the high limit trips to try and cool the unit down. Just a thought that crossed my mind.

[280plus]

Try and at least get us the brand and model #. You might have to pull the front cover to find the tag.

[Brad Johnson]

But if the temp limiter trips won't it reset when power is cut at the breaker?

Brad

[AZRedhawk44]

The filter was dirty, but I changed it yesterday.  I just tested this:

How is the filter in that thing? I want to say it sounds like a manual reset high temperature limit is tripped and that could be caused by a dirty filter. Some units will energize the fan when the high limit trips to try and cool the unit down. Just a thought that crossed my mind.

and:

But if the temp limiter trips won't it reset when power is cut at the breaker?

Brad

I just plugged the thermostat back in after killing the system via breaker this afternoon, and turned on the fan.  Then I turned it off.  Reacted fine.

Could it have been working so hard due to the dirty filter that it kept its own temperature up?  Even after changing the filter yesterday?  And the brief power outages during my testing didn't give it time to cool back down?

I'll get up in the attic tomorrow and do some model research.... but if the problem stays under control I will be less than thoroughly motivated.  With the temp cooling and my yard and house well shaded by old trees, I'm not likely to use the AC (and therefore the fan) until late next spring.  And we don't use heaters often here in Phoenix.

[Brad Johnson]

Without a meter, a diagram, and a couple minutes tracing circuits it's pretty hard to say.

It's definitely restricted to a circuit that affects the fan, but from there ... 

By the way, will the thermostats interchange? If so try swapping them and see if the problem follows the 'stat.

Brad

[280plus]

But if the temp limiter trips won't it reset when power is cut at the breaker

No, the manual limit I'm thinking of requires you to push a red button to reset. I was thinking maybe once power was cut it reset a fan delay timer that took a minute or so to kick the fan back in. That's all I could think of, something is calling the fan delay timer in, but what? Depends on how sophisticated the circuit is. Let's all pray it's not a Lennox. 

But if he didn't have to push any buttons it wasn't that particular limit anyways.

[Brad Johnson]

Hmmm... Could a power glitch cause the delay circuit to energize when the power came back on?

Brad

[280plus]

Hell if I know... 

[Zardozimo Oprah Bannedalas]

Flip the breaker. Problem solved. 

[280plus]

If that doesn't work get a bigger hammer. And some duct tape, of course.

[Nick1911]

Hell if I know... 

If that doesn't work get a bigger hammer. And some duct tape, of course.

So HVAC work really is just like auto mechanic work!